5.1 Source Channels for Digital Audio Consoles

January 24, 2007
Last time we looked at some general characteristics of digital consoles. Now we will look more closely at those specifically needed for surround sound mixing.

As Andrew Wild, vice president of marketing for Euphonix noted, "There are three areas of the console that have to fully cope with the surround signals--source channels, busses and monitoring."

Let's start with the source channels.

It may be stating the obvious, but the number of physical inputs to your surround sound digital mixing system (surround console for short) will increase dramatically compared to stereo or mono. Instead of one or two channels per source, you'll be dealing with five or six channels.

Start by taking a survey of your sources. How many of your existing stereo or mono sources are likely to become 5.1 (that's six channels) in the next year, two years, five years and beyond? How many new surround sources do you expect to add within the same timeframes? Consider the complement of sources not only for typical productions like nightly news, and also for the most complex ones like election night coverage.

Surround sources may be more readily available sooner than you may think. As Kevin Emmott, marketing coordinator of Calrec said, "the reality of high-definition programming is that it will continue to create more demand for 5.1 content. We already see our customers needing more and more signal paths to create 5.1 channels even though HD programming with 5.1 audio is only just taking off."


Of course, add up the expected stereo and mono sources as well. And don't forget graphics devices with their audio effects outputs. How many VTRs and server or EVS channels do you expect to use and how many audio channels are associated with each video channel? How many sources from routing switchers?

And don't discount surround-sound microphones either. While they may not be needed for many typical day-to-day productions, there can be times when they're the best choice. When counting talent mics, remember to include any backup mics.

If you don't wish to frame-out or card now for future source requirements, ask prospective console manufacturers how easy it is to add physical inputs to the console in the future. What are any limitations? How flexibly can a console handle your complement of analog and digital sources? And how easily can that ratio be changed in the future? (The same questions apply to outputs as well.)

Now that you've got a handle on the number of required physical inputs, how many faders (channel strips) are you going to need? A lot will depend on the topography of the console system.

A console could use six faders to accommodate one 5.1 source, or perhaps two stereo--for front and surround left and right; plus one or two mono--center channel and LFE channel if needed.

These faders can often be controlled as a group, but this scheme uses up a lot of channel strips for even a few surround sources, and you would end up needing a very large control surface.

One approach to reduce the number of faders is to use layers, letting one fader control multiple layers, which would be "hidden" underneath the fader. This keeps channel strip count to a reasonable number. But you'd want to make sure if there's enough layers to do the job, ascertain how easy it is to access the layers to make individual channel changes and how flexible the layers can be. Ask yourself how much scrolling through layers do you really want to do during a live production.

Today, many of the newer console systems offer flexibility in the way the channel strips can be operated. If you have a mono source, the strip is assigned to be mono. If stereo or surround, then the strip is assigned for these purposes. These assignments not only determine how many elements the fader controls, but also sets up the way the channel strip will function, like for panning and "spill."

Signal processing like gain, dynamics, equalization and delay, can be easily applied with a single set of controls to all of the channels comprising one source. But often, the audio operator needs to gain access to the individual elements as if they were independent channels. Perhaps the center channel needs boosting. Maybe the surround pair needs some brightening.


So here are some more things to consider when deciding on a surround console. How easy is it to handle the mix of mono, stereo, and surround sources, and how easy can this configuration be changed from show to show? Can fader strips be assigned to different types of sources, mono, stereo or surround? How easy is it to spill out the channels to make adjustments on individual elements? And what functions can be done while in spill mode?

To get a better idea of how this applies in real world consoles, let's hear from a couple of manufacturers.

First, Andrew Wild of Euphonix:

"The console must be able to cope with different format sources, each from a single channel strip and fader. The operator must be able to route, add EQ, or apply dynamics to all elements in the surround source from a single set of linked controls," he said. The spill feature "allows the operator to instantly spill out all six elements--L-C-R-Ls-Rs-LFE--of the 5.1 onto the console surface so that the individual elements may be adjusted if necessary. Metering of the channel should also be able to show all the six elements as a 5.1 meter. Euphonix allows the operator to work in mono, stereo, LCRS, 5.1 or 7.1 format. This is important, as there will always be differing number of different format sources for each show. The console must be flexible enough to allow the operator to work with any combination of sources encountered."

Next, Kevin Emmott from Calrec:

"A true surround-capable desk should provide comprehensive control of all the independent legs on a 5.1 channel. On a Calrec, surround channels use the resources of two stereo channels--for L/R and Ls/Rs--and two mono channels--for center and LFE. These resources will be allocated from the available mono and stereo channels as the surround channel is assigned, and can be individually controlled on a dedicated surround spill panel on the control surface.

"The advantage of using stereo channels for the L/R and Ls/Rs rather than four monos is that it allows faster operation when the signals need to be adjusted separately from the surround channel. It also ensures that during adjustment, the overall balance of the surround channel is not upset, for example, by adjusting the EQ of the left and then the right channel. It also allows adjustment of the front and rear width of the surround signal which would not be possible if they are treated as mono signals."

We'll pick this up next time with some discussion on panning, delay and other DSP functions.

(Thanks also to Niall Feldman of Solid State Logic and John Gluck of Calrec, for valuable input for this series on surround sound consoles.)

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